The invention relates to a color display tube with an envelope comprising
a neck portion, a funnel-shaped portion and a window portion;
an electron gun system arranged in the neck portion;
an elongated display screen having a phosphor pattern on the inner surface of the window portion;
a color selection means arranged opposite the display screen;
an internal magnetic shield arranged inside the funnel-shaped portion, which magnetic shield comprises two long side walls extending parallel to the long axis of the display screen (the x axis), two short side walls extending parallel to the short axis of the display screen (the y axis), and, on the side of the electron gun, an aperture allowing electrons to pass, which aperture extends transversely to the longitudinal axis of the display tube.
The term xe2x80x9ccolor selection meansxe2x80x9d is to be taken to mean herein, for example, a shadow mask plate provided with apertures, or a wire mask.
In a (color) display tube, the earth""s magnetic field causes deflection of the electron paths, which, without countermeasures, may be so substantial that the electrons impinge on a wrong phosphor (mislanding), leading to discoloration of the image. Particularly the component of the earth""s field in the axial direction of the display tube (the so-called axial field) is important in that it may manifest itself as a lack of color or even as color impurities in the corners of the display screen.
A well-known measure of reducing mislandings caused by the earth""s magnetic field is the use of an internal magnetic shield. The shape of such a shield broadly follows the contours of the envelope of the display tube. This means that the (funnel-shaped) shield comprises two long, more or less trapezoidal sides extending parallel to the long axis (the x axis) of the display screen, and two short, more or less trapezoidal sides extending parallel to the short axis (the y axis) of the display screen.
In many types of color display tubes, this shield consists of an iron (bath) tub deep-drawn from strip material, which tub is provided with one or more functional openings. A drawback of the deep-drawn product resides in that a fairly substantial part of the strip material has to be disposed of as scrap. Other types of color display tubes often comprise a magnetic shield which is cut from a flat sheet and subsequently bent. In the case of the bent shield, the percentage of scrap is often smaller than in the deep-drawn shield, however, the necessary additional operations, such as spot welding or manual bending of fastening lugs, lead to higher costs. Bent shields are employed in display tubes in various embodiments, however, there is a trend towards deep-drawn shields because they can be manufactured more economically in large numbers. In view of the difficult competitive position of display tubes, in particular with respect to LCDs, a still further reduction in costs is required. Therefore, it is an object of the invention to provide an embodiment of a (deep-drawn) shield which can be manufactured more economically while its magnetic performance is maintained.
To achieve this, a display tube of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the shield is a tub drawn from a sheet or strip material (slug), the bottom of the tub being provided with a central opening for allowing electrons to pass, and material of the bottom adjoining the central opening being bent outwards from the plane of the bottom.
The invention enables the costs of the shield to be minimized while the desired magnetic shielding performance is maintained. The cost reduction can be achieved by reducing the depth of draw of the bath tub, as a result of which a smaller slug from the strip material is needed. The part of the bottom that remains after cutting out the opening allowing electrons to pass is often referred to as the shield diaphragm. In the invention, the shield diaphragm cut from the bottom of the tub is smaller than the opening for allowing passage of electrons in the bottom of the tub, whereafter the excess part is bent outwards such that corner cutting of the electron beam cannot take place.
A practical embodiment of a display tube in accordance with the invention is characterized in that the opening allowing passage of electrons in the shield is elongated and includes a pair of opposite long sides, and in that strips of the material of the bottom adjoining the long sides of the opening are bent outwards from the plane of the bottom.
In certain types of display tubes it is advantageous if strips of the material of the bottom are bent outwards along both the long and the short sides of the opening.
The above-mentioned measures enable the effect of an optimum shield height to be achieved, using, for example, a 20% smaller slug and a smaller shield height, as a result of which optimum magnetic shielding is achieved using 20% less material.
The use of a slug which is more than 20% smaller than the customary slug, particularly a slug which is more than 25% smaller, generally no longer leads to the desired result. In practice, 10 to 15% smaller slugs proved very suitable, both from the viewpoint of material economy and magnetic shielding. In addition, the use of said slugs enables the total design to be realized in a simple manner, whereby corner cutting of the electrons is precluded.
The display screen is generally elongated and so is the opening allowing passage of electrons in the bottom of the shield, i.e. said opening has two long sides and two short sides. In the case of display tubes having a line mask structure, i.e. the phosphor pattern is a line pattern, preferably strips of the bottom material are bent which adjoin the long sides of the opening allowing passage of electrons. In said display tubes the measure in accordance with the invention is most effective. At smaller formats, such as 14xe2x80x3 tubes and 20 V tubes, the invention is particularly attractive. In the case of display tubes having a hexagonal mask structure, where the phosphor pattern is a pattern of dots, preferably, strips of the bottom material are bent which adjoin the short sides of the opening allowing passage of electrons.
The angles through which the material strips are bent from the plane of the bottom also is an important parameter. This angle, indicated in the Figures by means of xcex2, should preferably be smaller than the maximum deflection angle, indicated in the Figures by means of a, including overscan. Preferably 5 to 30 degrees smaller. Within this range, the size depends, inter alia, on the depth of the tube (the depth of a 90xc2x0 tube exceeds that of a 110xc2x0 tube).
Further preferred embodiments are:
the smallest distance between the material strips bent from the plane of the bottom, indicated in the Figures by means of Wtop, is at least 5% smaller than the maximum width of the diaphragm opening, indicated in the Figures by means of Wmax.
the total height of the shield, indicated in the Figures by means of Htot, is at least 10% larger than the height of the body of the tub, indicated in the Figures by means of Hbod. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.